The present invention relates to a gas-insulated circuit breaker, and in particular, it relates to a circuit breaker having a parallel resistor for suppressing the occurrence of arc when making contact.
When closing a non-loaded power transmission line by means of a circuit breaker, it has been known to use a circuit breaker having a parallel resistor in order to suppress an overvoltage which occurs as a closing switching surge. An equivalent circuit thereof is shown in FIG. 3, and a timing chart indicative of closure and interruption of the circuit breaker is shown in FIG. 4, respectively.
This circuit breaker is comprised of a main contact S1 that has a current interruption capability, a resistor 16 and a resistor closing contact S2 both coupled in parallel with the main contact.
When closing the circuit breaker, resistor closing contact S2 is closed at time t1 preceding time t2 at which the main contact S1 is to be closed so that resistor 16 is inserted into the circuit to cause a preliminary discharge to occur through the resistor closing contact S2 in precedence. This method is widely applied for suppressing closing switching surges, since when it is applied, for example, to a 500 kV power transmission system, a value of multiple of overvoltages imposed on closing of the circuit breaker can be limited to less than 1.7 by setting a value of resistor at several hundred ohms and a time difference at approximately 0.5 cycle between the closures of the resistor closing contact and the main contact.
On the other hand, since the resistor closing contact S2 has little current interruption capability, at the time of interrupting operation of the main contact S1, the resistor closing contact S1 must be opened at time t3 preceding time t4 at which the main contact S1 is opened to effect interruption in order to ensure inter-electrode isolation of the resistor closing contact S2 to be maintained, thereby requiring a different operational characteristic from that at the time of the closing operation.
One typical example of such prior art circuit breakers with parallel resistor is shown in FIG. 9. FIG. 9(a) is a cross-sectional view in part of a schematic construction thereof, and FIG. 9(b) is a cross-sectional view of a resistor closing contact S2 for use therein in its full open state (at its maximum distance).
In the drawings of FIG. 9, main contact S1 and resistor closing contact S2 are disposed inside a hermetically sealed chamber (not shown) filled with arc-extinction gas.
The main contact S1 that has a current interruption capability comprises a stationary unit 31 and a movable unit 41. The resistor closing contact S2 comprises a stationary unit 11' and a movable unit 21'.
Stationary unit 31 on the side of the main contact S1 comprises a stationary contact 32 and an electric field relaxation shield 33 that surrounds the stationary contact 32. On the other hand, the movable unit 41 on the side of the main contact S1 includes a movable contact 42 attached to a cylinder 44. The cylinder 44 and a piston 45 constitute a gas compression unit which responsive to interrupting (opening) operation of both contacts 32 and 42 compresses a filled gas and blows it between the contacts to extinguish arc through an insulation nozzle 43.
The stationary unit 11' on the side of the resistor closing contact S2, which is firmly attached to the stationary unit 31 on the side of the main contact S1 via a support fixture 34, comprises a stationary electrode 12, a stationary shield 13, a resistor 16 coupled to the stationary electrode 12 via a conducting support member 15, and the like, wherein the stationary electrode 12 is supported by the support member 15 via a spring 14. On the other hand, the movable unit 21' on the side of the resistor closing contact S2 includes a movable electrode 22' which is supported by a support fixture 27 such that it can move integral with the movable unit 41 on the side of the main contact S1, and a shield 23' therefor, wherein the movable electrode 22' is coupled via its axial member 26' and coupling member 27 to the movable unit 41 on the side of the main contact S1.
At the time of closing of the circuit breaker, the movable unit 21' of the resistor closing contact S2 is directed toward the stationary unit 11' thereof integral with the movement of the movable unit 41 of the main contact S1. Since an inter-electrode length 10 in a full open state (maximum distance) of the resistor closing contact S2 is set shorter than an inter-electrode length of the main contact S1, the resistor closing contact S2 is caused to close at first with its movable electrode 22' further pushing the stationary electrode 12 inward by a distance 1.sub.w against the force of a spring 14, then, the main contact S1 is closed.
On the other hand, at the time of interruption of the circuit breaker, the movable unit 41 of the main contact S1 moves backward with its contacts 32 and 42 somewhat being maintained in contact. The movable unit 21' of the resistor closing contact S2 (that is, movable electrode 22' and its shield 23') which is adapted to move integral with the movable unit 41 is caused to move in the open direction at a speed of interruption of the main contact S1, however, since the spring 14 cannot follow the speed of interruption, thus the stationary electrode 12 is caused to return at a slower speed, thereby, the resistor closing contact S2 can be opened in precedence to the opening of the main contact S1.
In addition to the above-mentioned prior art, there are still other prior art circuit breakers which have employed an actuating mechanism for separately actuating the main contact and the resistor closing contact, or modified the construction of the resistor closing contact, as disclosed in JP-A-Nos. 1-246732, 3-4418, 3-297021, 4-286822.
As a duty of the resistor closing contact, it is required at the time of interrupting operation that a sufficiently higher electric field relative to that at the main contact is formed around the resistor closing contact to ensure a preceding discharge to occur, then insert the resistor in the system. On the other hand, at the time of interrupting operation, it is required to provide an appropriate structure to adequately shield the contact electrodes so as to prevent electric field concentration, and which can withstand a large transient recovery voltage which appears between the electrodes immediately upon onset of interrupting operation, thereby, a quite different characteristic in contrast with that required at the time of closing must be satisfied as well. In addition, the resistor closing contact is required to have an insulation performance as high as that of the main contact at its full open state in spite of its shorter inter-electrode distance than that of the main contact.
Along with an increasing voltage in the power transmission system nowadays, a compacter design of circuit breakers is under way, thus, insulation coordination characteristics imposed thereon are getting more and more stringent. Because of increasing difficulties by the prior art arrangements to cope with such stringent requirements, a method to operate the resistor closing contact independently has been devised as described above, however, this method would inevitably results in a complicated actuator control system.
Further, the circuit breaker described in JP-A No. 1-246732 discloses that during its closing operational stroke, the movable electrode of its resistor closing contact is protruded outside its shield, and that during its interrupting operational stroke, the movable electrode thereof is retracted inside the shield so as to improve the insulation coordination characteristics. However, there are such problems associated with the prior art circuit breakers that the internal structure of a movable unit of its resistor closing contact becomes more complicated requiring an increased number of components and parts, as well as that since the movable electrode which has been retracted inside the shield is pushed to protrude outside thereof by a push bar when it arrives at its maximum point of distance and so is set ready for the next closing operation as protruded, therefore, requiring an additional distance for its maximum inter-electrode distance to ensure adequate insulation therebetween, which is contrary to the general requirements to shorten the inter-electrode distance to manufacture a compacter circuit breaker.